Method for processing image on basis of intra prediction mode and apparatus therefor

ABSTRACT

In the present invention, a method for processing an image on the basis of an intra prediction mode and an apparatus therefor are disclosed. Particularly, the method for processing an image on the basis of an intra prediction mode may comprise the steps of: partitioning a block to be processed, on the basis of an intra prediction mode of the block to be processed; and performing intra prediction for the split block to be processed, wherein the direction in which the block to be processed is split is perpendicular to the prediction direction of the intra prediction mode of the block to be processed.

CROSS-REFERENCE TO RELATED APPLICATIONS

This application is the National Stage filing under 35 U.S.C. 371 ofInternational Application No. PCT/KR2015/013170, filed on Dec. 3, 2015,which claims the benefit of U.S. Provisional Application No. 62/137,163,filed on Mar. 23, 2015 the contents of which are all hereby incorporatedby reference herein in their entirety.

TECHNICAL FIELD

The present invention relates to a method for processing a still imageor moving image and, more particularly, to a method forencoding/decoding a still image or moving image based on anintra-prediction mode and an apparatus supporting the same.

BACKGROUND ART

Compression encoding means a series of signal processing techniques fortransmitting digitized information through a communication line ortechniques for storing information in a form suitable for a storagemedium. The medium including a picture, an image, audio, etc. may be atarget for compression encoding, and particularly, a technique forperforming compression encoding on a picture is referred to as videoimage compression.

Next-generation video contents are supposed to have the characteristicsof high spatial resolution, a high frame rate and high dimensionality ofscene representation. In order to process such contents, a drasticincrease in the memory storage, memory access rate and processing powerwill result.

Accordingly, it is required to design a coding tool for processingnext-generation video contents efficiently.

DISCLOSURE Technical Problem

In the existing compression technology of a still image or moving image,a method of compressing an image based on a block is used, but thecharacteristics of an image may not be properly reflected because theimage is split and compressed in a fixed form of a square form in theimage compression technology based on a block. In particular, when anintra-prediction mode is applied, there is a problem in that predictionaccuracy is reduced as the distance from a reference sample becomesdistant.

In order to solve such a problem, an object of the present invention isto propose a method of splitting an image based on the direction of anintra-prediction mode.

Furthermore, an object of the present invention is to propose a methodof performing encoding/decoding on a split block based on the directionof an intra-prediction mode.

Furthermore, an object of the present invention is to propose a methodof reconstructing a split block in a square block form based on thedirection of an intra-prediction mode.

The objects of the present invention are not limited to the technicalobjects described above, and other technical that are objects notmentioned herein may be understood to those skilled in the art from thedescription below.

Technical Solution

In an aspect of the present invention, a method of processing an imagebased on an intra-prediction mode includes the steps of splitting aprocessing block based on an intra-prediction mode of the processingblock and performing intra-prediction on the split processing block,wherein a split direction of the split processing block may be verticalto the prediction direction of the intra-prediction mode of theprocessing block.

In an aspect of the present invention, an apparatus for processing animage based on an intra-prediction mode includes a split unit splittinga processing block based on an intra-prediction mode of the processingblock and an intra-prediction processing unit performingintra-prediction on the split processing block, wherein a splitdirection of the split processing block may be vertical to theprediction direction of the intra-prediction mode of the processingblock.

Preferably, the processing block may be split when a split flag of theprocessing block is 1.

Preferably, whether the processing block is split according to a squarequad-tree method or split vertically to the prediction direction may bedetermined based on the intra-prediction mode of the processing block.

Preferably, the processing block may be split according to the squarequad-tree method if the intra-prediction mode of the processing block isintra-planar or intra-DC, otherwise, the processing block may be splitvertically to the prediction direction.

Preferably, the method may further include the steps of reconstructingthe split processing block into a square block and performingtransform/inverse transform on the reconstructed processing block.

Preferably, if the split processing block is 2N×N/2, the splitprocessing block may be split into two blocks each having a halfhorizontal size and the two blocks may be vertically relocated toreconstruct a square N×N block.

Preferably, if the split processing block is N/2×2N, the splitprocessing block may be split into two blocks each having a halfvertical size and the two blocks may be horizontally relocated toreconstruct a square N×N block.

Preferably, samples included in the split processing block may berelocated in predetermined order to reconstruct the square block.

Preferably, the method may further include the step of constructing areference sample for the split processing block.

Preferably, if the split direction of the split processing block ishorizontal or vertical, the reference sample may include a sampleneighboring a left boundary of the split processing block, a sampleneighboring a top boundary of the split processing block and a sampleneighboring a top left of the split processing block.

Preferably, if the split direction of the processing block is 45°, thereference sample may include a sample neighboring a left boundary of thesplit processing block, a sample neighboring a top boundary of the splitprocessing block, and a sample neighboring a top left of the splitprocessing block.

Preferably, if the split direction of the processing block is 45°, thereference sample may include a sample neighboring a top left boundary ofthe split processing block, a sample neighboring a right boundary of thesplit processing block, a sample neighboring a bottom boundary of thesplit processing block.

Advantageous Effects

In accordance with an embodiment of the present invention, a still imageor moving image is split based on the direction of an intra-predictionmode. Accordingly, prediction accuracy can be improved because thedistance between a reference sample and a prediction sample is reducedwhen intra-prediction is applied.

Furthermore, in accordance with an embodiment of the present invention,transform/inverse transform can be performed using a previously definedtransform/inverse transform scheme by relocating (or reconstructing) asplit block in a square block form based on the direction of anintra-prediction mode.

The technical effects of the present invention are not limited to thetechnical effects described above, and other technical effects notmentioned herein may be understood to those skilled in the art from thedescription below.

DESCRIPTION OF DRAWINGS

The accompanying drawings, which are included herein as a part of thedescription for help understanding the present invention, provideembodiments of the present invention, and describe the technicalfeatures of the present invention with the description below.

FIG. 1 is illustrates a schematic block diagram of an encoder in whichthe encoding of a still image or video signal is performed, as anembodiment to which the present invention is applied.

FIG. 2 illustrates a schematic block diagram of a decoder in whichdecoding of a still image or video signal is performed, as an embodimentto which the present invention is applied.

FIG. 3 is a diagram for describing a split structure of a coding unitthat may be applied to the present invention.

FIG. 4 is a diagram for describing a prediction unit that may be appliedto the present invention.

FIG. 5 is an embodiment to which the present invention is applied and isa diagram illustrating an intra-prediction method.

FIG. 6 illustrates a prediction direction according to anintra-prediction mode.

FIGS. 7 and 8 are diagrams for illustrating a problem in existingintra-mode prediction.

FIG. 9 illustrates an intra-prediction mode-based spit method accordingto an embodiment of the present invention.

FIG. 10 illustrates a method of constructing a reference sample for asplit block based on an intra-prediction mode according to an embodimentof the present invention.

FIG. 11 is a diagram for illustrating a comparison between an existingblock split method and the block intra-prediction mode-based spit methodaccording to the present invention.

FIG. 12 is a more detailed diagram illustrating an intra-prediction unitaccording to an embodiment of the present invention.

FIGS. 13 and 14 are diagrams illustrating a method of processing a videosignal based on an intra-prediction mode according to an embodiment ofthe present invention.

FIG. 15 is a diagram for illustrating a method of relocating (orreconstructing) a transform unit according to an embodiment of thepresent invention.

FIG. 16 is a diagram for illustrating a comparison between an existingtransform block split method and a transform block reconstruction methodaccording to the present invention.

FIG. 17 is a diagram for illustrating a method of relocating (orreconstructing) a transform unit according to an embodiment of thepresent invention.

FIG. 18 is a diagram for illustrating a method of relocating (orreconstructing) a transform unit according to an embodiment of thepresent invention.

FIG. 19 is a more detailed diagram illustrating a transform unit/inversetransform unit according to an embodiment of the present invention.

FIG. 20 is a diagram illustrating a method of processing a video signalbased on an intra-prediction mode according to an embodiment of thepresent invention.

MODE FOR INVENTION

Hereinafter, a preferred embodiment of the present invention will bedescribed by reference to the accompanying drawings. The descriptionthat will be described below with the accompanying drawings is todescribe exemplary embodiments of the present invention, and is notintended to describe the only embodiment in which the present inventionmay be implemented. The description below includes particular details inorder to provide perfect understanding of the present invention.However, it is understood that the present invention may be embodiedwithout the particular details to those skilled in the art.

In some cases, in order to prevent the technical concept of the presentinvention from being unclear, structures or devices which are publiclyknown may be omitted, or may be depicted as a block diagram centering onthe core functions of the structures or the devices.

Further, although general terms widely used currently are selected asthe terms in the present invention as much as possible, a term that isarbitrarily selected by the applicant is used in a specific case. Sincethe meaning of the term will be clearly described in the correspondingpart of the description in such a case, it is understood that thepresent invention will not be simply interpreted by the terms only usedin the description of the present invention, but the meaning of theterms should be figured out.

Specific terminologies used in the description below may be provided tohelp the understanding of the present invention. Furthermore, thespecific terminology may be modified into other forms within the scopeof the technical concept of the present invention. For example, asignal, data, a sample, a picture, a frame, a block, etc may be properlyreplaced and interpreted in each coding process.

Hereinafter, in this specification, a “processing unit” means a unit inwhich an encoding/decoding processing process, such as prediction,transform and/or quantization, is performed. Hereinafter, forconvenience of description, a processing unit may also be called a“processing block” or “block.”

A processing unit may be construed as having a meaning including a unitfor a luma component and a unit for a chroma component. For example, aprocessing unit may correspond to a coding tree unit (CTU), a codingunit (CU), a prediction unit (PU) or a transform unit (TU).

Furthermore, a processing unit may be construed as being a unit for aluma component or a unit for a chroma component. For example, theprocessing unit may correspond to a coding tree block (CTB), codingblock (CB), prediction block (PB) or transform block (TB) for a lumacomponent. Alternatively, a processing unit may correspond to a codingtree block (CTB), coding block (CB), prediction block (PB) or transformblock (TB) for a chroma component. Furthermore, the present invention isnot limited thereto, and a processing unit may be construed as a meaningincluding a unit for a luma component and a unit for a chroma component.

Furthermore, a processing unit is not essentially limited to a squareblock and may be constructed in a polygon form having three or morevertices.

Furthermore, hereinafter, in this specification, a pixel, a pictureelement, etc. are collectively called a sample. Furthermore, to use asample may mean to use a pixel value, a picture element value or thelike.

FIG. 1 is illustrates a schematic block diagram of an encoder in whichthe encoding of a still image or video signal is performed, as anembodiment to which the present invention is applied.

Referring to FIG. 1, the encoder 100 may include a video split unit 110,a subtractor 115, a transform unit 120, a quantization unit 130, adequantization unit 140, an inverse transform unit 150, a filtering unit160, a decoded picture buffer (DPB) 170, a prediction unit 180 and anentropy encoding unit 190. Furthermore, the prediction unit 180 mayinclude an inter-prediction unit 181 and an intra-prediction unit 182.

The video split unit 110 splits an input video signal (or picture orframe), input to the encoder 100, into one or more processing units.

The subtractor 115 generates a residual signal (or residual block) bysubtracting a prediction signal (or prediction block), output by theprediction unit 180 (i.e., by the inter-prediction unit 181 or theintra-prediction unit 182), from the input video signal. The generatedresidual signal (or residual block) is transmitted to the transform unit120.

The transform unit 120 generates transform coefficients by applying atransform scheme (e.g., discrete cosine transform (DCT), discrete sinetransform (DST), graph-based transform (GBT) or Karhunen-Loeve transform(KLT)) to the residual signal (or residual block). In this case, thetransform unit 120 may generate transform coefficients by performingtransform using a prediction mode applied to the residual block and atransform scheme determined based on the size of the residual block.

In particular, the transform unit 120 according to the present inventionmay perform transform by reconstructing a processing block in a squareblock if a current processing block is not a square block. The transformunit 120 is described in detail later.

The quantization unit 130 quantizes the transform coefficient andtransmits it to the entropy encoding unit 190, and the entropy encodingunit 190 performs an entropy coding operation of the quantized signaland outputs it as a bit stream.

Meanwhile, the quantized signal outputted by the quantization unit 130may be used to generate a prediction signal. For example, a residualsignal may be reconstructed by applying dequatization and inversetransformation to the quantized signal through the dequantization unit140 and the inverse transform unit 150. A reconstructed signal may begenerated by adding the reconstructed residual signal to the predictionsignal output by the inter-prediction unit 181 or the intra-predictionunit 182.

Meanwhile, during such a compression process, neighbor blocks arequantized by different quantization parameters. Accordingly, an artifactin which a block boundary is shown may occur. Such a phenomenon isreferred to a blocking artifact, which is one of important factors forevaluating image quality. In order to decrease such an artifact, afiltering process may be performed. Through such a filtering process,the blocking artifact is removed and the error of a current picture isdecreased at the same time, thereby improving image quality.

The filtering unit 160 applies filtering to the reconstructed signal,and outputs it through a playback device or transmits it to the decodedpicture buffer 170. The filtered signal transmitted to the decodedpicture buffer 170 may be used as a reference picture in theinter-prediction unit 181. As described above, an encoding rate as wellas image quality can be improved using the filtered picture as areference picture in an inter-picture prediction mode.

The decoded picture buffer 170 may store the filtered picture in orderto use it as a reference picture in the inter-prediction unit 181.

The inter-prediction unit 181 performs temporal prediction and/orspatial prediction with reference to the reconstructed picture in orderto remove temporal redundancy and/or spatial redundancy. In this case, ablocking artifact or ringing artifact may occur because a referencepicture used to perform prediction is a transformed signal thatexperiences quantization or dequantization in a block unit when it isencoded/decoded previously.

Accordingly, in order to solve performance degradation attributable tothe discontinuity of such a signal or quantization, signals betweenpixels may be interpolated in a sub-pixel unit by applying a low passfilter to the inter-prediction unit 181. In this case, the sub-pixelmeans a virtual pixel generated by applying an interpolation filter, andan integer pixel means an actual pixel that is present in areconstructed picture. A linear interpolation, a bi-linearinterpolation, a wiener filter, and the like may be applied as aninterpolation method.

The interpolation filter may be applied to the reconstructed picture,and may improve the accuracy of prediction. For example, theinter-prediction unit 181 may perform prediction by generating aninterpolation pixel by applying the interpolation filter to the integerpixel and by using the interpolated block including interpolated pixelsas a prediction block.

The intra-prediction unit 182 predicts a current block with reference tosamples neighboring the block that is now to be encoded. Theintra-prediction unit 182 may perform the following procedure in orderto perform intra-prediction. First, the intra-prediction unit 182 mayprepare a reference sample necessary to generate a prediction signal.Furthermore, the intra-prediction unit 182 may generate a predictionsignal using the prepared reference sample. Next, the intra-predictionunit 182 may encode a prediction mode. In this case, the referencesample may be prepared through reference sample padding and/or referencesample filtering. A quantization error may be present because thereference sample experiences the prediction and the reconstructionprocess. Accordingly, in order to reduce such an error, a referencesample filtering process may be performed on each prediction mode usedfor the intra-prediction.

In particular, the intra-prediction unit 182 according to the presentinvention may split a current processing block based on a splitdirection determined based on an intra-prediction mode, and may performintra-prediction on the split processing blocks. The intra-predictionunit 182 is described in detail later.

The prediction signal (or prediction block) generated through theinter-prediction unit 181 or the intra-prediction unit 182 may be usedto generate a reconstructed signal (or reconstructed block) or may beused to generate a residual signal (or residual block).

FIG. 2 illustrates a schematic block diagram of a decoder in whichdecoding of a still image or video signal is performed, as an embodimentto which the present invention is applied.

Referring to FIG. 2, the decoder 200 may include an entropy decodingunit 210, a dequantization unit 220, an inverse transform unit 230, anadder 235, a filtering unit 240, a decoded picture buffer (DPB) 250 anda prediction unit 260. Furthermore, the prediction unit 260 may includean inter-prediction unit 261 and an intra-prediction unit 262.

Furthermore, a reconstructed video signal output through the decoder 200may be played back through a playback device.

The decoder 200 receives a signal (i.e., bit stream) output by theencoder 100 shown in FIG. 1. The entropy decoding unit 210 performs anentropy decoding operation on the received signal.

The dequantization unit 220 obtains transform coefficients from theentropy-decoded signal using quantization step size information.

The inverse transform unit 230 obtains a residual signal (or residualblock) by inverse transforming the transform coefficients by applying aninverse transform scheme.

In particular, the inverse transform unit 230 according to the presentinvention may perform inverse transform by reconstructing a currentprocessing block in a square block if the current processing block isnot a square block. The inverse transform unit 230 is described indetail later.

The adder 235 adds the obtained residual signal (or residual block) tothe prediction signal (or prediction block) output by the predictionunit 260 (i.e., the inter-prediction unit 261 or the intra-predictionunit 262), thereby generating a reconstructed signal (or reconstructedblock).

The filtering unit 240 applies filtering to the reconstructed signal (orreconstructed block) and outputs the filtered signal to a playbackdevice or transmits the filtered signal to the decoded picture buffer250. The filtered signal transmitted to the decoded picture buffer 250may be used as a reference picture in the inter-prediction unit 261.

In this specification, the embodiments described in the filtering unit160, inter-prediction unit 181 and intra-prediction unit 182 of theencoder 100 may be identically applied to the filtering unit 240,inter-prediction unit 261 and intra-prediction unit 262 of the decoder,respectively.

In particular, the intra-prediction unit 262 according to the presentinvention may split a current processing block based on a splitdirection determined based on an intra-prediction mode, and may performintra-prediction on the split processing block. The intra-predictionunit 262 is described in detail later.

In general, a block-based image compression method is used in thecompression technique (e.g., HEVC) of a still image or a video. Theblock-based image compression method is a method of processing an imageby splitting it into specific block units, and may decrease memory useand a computational load.

FIG. 3 is a diagram for describing a split structure of a coding unitwhich may be applied to the present invention.

An encoder splits a single image (or picture) into coding tree units(CTUs) of a quadrangle form, and sequentially encodes the CTUs one byone according to raster scan order.

In HEVC, a size of CTU may be determined as one of 64×64, 32×32, and16×16. The encoder may select and use the size of a CTU based onresolution of an input video signal or the characteristics of inputvideo signal. The CTU includes a coding tree block (CTB) for a lumacomponent and the CTB for two chroma components that correspond to it.

One CTU may be split in a quad-tree structure. That is, one CTU may besplit into four units each having a square form and having a halfhorizontal size and a half vertical size, thereby being capable ofgenerating coding units (CUs). Such splitting of the quad-tree structuremay be recursively performed. That is, the CUs are hierarchically splitfrom one CTU in the quad-tree structure.

A CU means a basic unit for the processing process of an input videosignal, for example, coding in which intra/inter prediction isperformed. A CU includes a coding block (CB) for a luma component and aCB for two chroma components corresponding to the luma component. InHEVC, a CU size may be determined as one of 64×64, 32×32, 16×16, and8×8.

Referring to FIG. 3, the root node of a quad-tree is related to a CTU.The quad-tree is split until a leaf node is reached. The leaf nodecorresponds to a CU.

This is described in more detail. The CTU corresponds to the root nodeand has the smallest depth (i.e., depth=0) value. A CTU may not be splitdepending on the characteristics of an input video signal. In this case,the CTU corresponds to a CU.

A CTU may be split in a quad-tree form. As a result, lower nodes, thatis, a depth 1 (depth=1), are generated. Furthermore, a node (i.e., leafnode) that belongs to the lower nodes having the depth of 1 and that isno longer split corresponds to a CU. For example, in FIG. 3(b), a CU(a),a CU(b) and a CU(j) corresponding to nodes a, b and j have been oncesplit from the CTU, and have a depth of 1.

At least one of the nodes having the depth of 1 may be split in aquad-tree form. As a result, lower nodes having a depth 1 (i.e.,depth=2) are generated. Furthermore, a node (i.e., leaf node) thatbelongs to the lower nodes having the depth of 2 and that is no longersplit corresponds to a CU. For example, in FIG. 3(b), a CU(c), a CU(h)and a CU(i) corresponding to nodes c, h and i have been twice split fromthe CTU, and have a depth of 2.

Furthermore, at least one of the nodes having the depth of 2 may besplit in a quad-tree form again. As a result, lower nodes having a depth3 (i.e., depth=3) are generated. Furthermore, a node (i.e., leaf node)that belongs to the lower nodes having the depth of 3 and that is nolonger split corresponds to a CU. For example, in FIG. 3(b), a CU(d), aCU(e), a CU(f) and a CU(g) corresponding to nodes d, e, f and g havebeen three times split from the CTU, and have a depth of 3.

In the encoder, a maximum size or minimum size of a CU may be determinedbased on the characteristics of a video image (e.g., resolution) or byconsidering the encoding rate. Furthermore, information about themaximum or minimum size or information capable of deriving theinformation may be included in a bit stream. A CU having a maximum sizeis referred to as the largest coding unit (LCU), and a CU having aminimum size is referred to as the smallest coding unit (SCU).

In addition, a CU having a tree structure may be hierarchically splitwith predetermined maximum depth information (or maximum levelinformation). Furthermore, each split CU may have depth information.Since the depth information represents a split count and/or degree of aCU, it may include information about the size of a CU.

Since the LCU is split in a Quad-tree shape, the size of SCU may beobtained by using a size of LCU and the maximum depth information. Or,inversely, the size of LCU may be obtained by using a size of SCU andthe maximum depth information of the tree.

For a single CU, the information (e.g., a split CU flag (split cu flag))that represents whether the corresponding CU is split may be forwardedto the decoder. This split information is included in all CUs except theSCU. For example, when the value of the flag that represents whether tosplit is ‘1’, the corresponding CU is further split into four CUs, andwhen the value of the flag that represents whether to split is ‘0’, thecorresponding CU is not split any more, and the processing process forthe corresponding CU may be performed.

As described above, a CU is a basic unit of the coding in which theintra-prediction or the inter-prediction is performed. The HEVC splitsthe CU in a prediction unit (PU) for coding an input video signal moreeffectively.

A PU is a basic unit for generating a prediction block, and even in asingle CU, the prediction block may be generated in different way by aunit of PU. However, the intra-prediction and the inter-prediction arenot used together for the PUs that belong to a single CU, and the PUsthat belong to a single CU are coded by the same prediction method(i.e., the intra-prediction or the inter-prediction).

A PU is not split in the Quad-tree structure, but is split once in asingle CU in a predetermined shape. This will be described by referenceto the drawing below.

FIG. 4 is a diagram for describing a prediction unit that may be appliedto the present invention.

A PU is differently split depending on whether the intra-prediction modeis used or the inter-prediction mode is used as the coding mode of theCU to which the PU belongs.

FIG. 4(a) illustrates a PU if the intra-prediction mode is used, andFIG. 4(b) illustrates a PU if the inter-prediction mode is used.

Referring to FIG. 4(a), assuming that the size of a single CU is 2N×2N(N=4, 8, 16 and 32), the single CU may be split into two types (i.e.,2N×2N or N×N).

In this case, if a single CU is split into the PU of 2N×2N shape, itmeans that only one PU is present in a single CU.

Meanwhile, if a single CU is split into the PU of N×N shape, a single CUis split into four PUs, and different prediction blocks are generatedfor each PU unit. However, such PU splitting may be performed only ifthe size of CB for the luma component of CU is the minimum size (i.e.,the case that a CU is an SCU).

Referring to FIG. 4(b), assuming that the size of a single CU is 2N×2N(N=4, 8, 16 and 32), a single CU may be split into eight PU types (i.e.,2N×2N, N×N, 2N×N, N×2N, nL×2N, nR×2N, 2N×nU and 2N×nD)

As in the intra-prediction, the PU split of N×N shape may be performedonly if the size of CB for the luma component of CU is the minimum size(i.e., the case that a CU is an SCU).

The inter-prediction supports the PU split in the shape of 2N×N that issplit in a horizontal direction and in the shape of N×2N that is splitin a vertical direction.

In addition, the inter-prediction supports the PU split in the shape ofnL×2N, nR×2N, 2N×nU and 2N×nD, which is an asymmetric motion split(AMP). In this case, ‘n’ means ¼ value of 2N. However, the AMP may notbe used if the CU to which the PU is belonged is the CU of minimum size.

In order to encode the input video signal in a single CTU efficiently,the optimal split structure of the coding unit (CU), the prediction unit(PU) and the transform unit (TU) may be determined based on a minimumrate-distortion value through the processing process as follows. Forexample, as for the optimal CU split process in a 64×64 CTU, therate-distortion cost may be calculated through the split process from aCU of 64×64 size to a CU of 8×8 size. The detailed process is asfollows.

1) The optimal split structure of a PU and TU that generates the minimumrate distortion value is determined by performinginter/intra-prediction, transformation/quantization,dequantization/inverse transformation and entropy encoding on the CU of64×64 size.

2) The optimal split structure of a PU and TU is determined to split the64×64 CU into four CUs of 32×32 size and to generate the minimum ratedistortion value for each 32×32 CU.

3) The optimal split structure of a PU and TU is determined to furthersplit the 32×32 CU into four CUs of 16×16 size and to generate theminimum rate distortion value for each 16×16 CU.

4) The optimal split structure of a PU and TU is determined to furthersplit the 16×16 CU into four CUs of 8×8 size and to generate the minimumrate distortion value for each 8×8 CU.

5) The optimal split structure of a CU in the 16×16 block is determinedby comparing the rate-distortion value of the 16×16 CU obtained in theprocess 3) with the addition of the rate-distortion value of the four8×8 CUs obtained in the process 4). This process is also performed forremaining three 16×16 CUs in the same manner.

6) The optimal split structure of CU in the 32×32 block is determined bycomparing the rate-distortion value of the 32×32 CU obtained in theprocess 2) with the addition of the rate-distortion value of the four16×16 CUs that is obtained in the process 5). This process is alsoperformed for remaining three 32×32 CUs in the same manner.

7) Finally, the optimal split structure of CU in the 64×64 block isdetermined by comparing the rate-distortion value of the 64×64 CUobtained in the process 1) with the addition of the rate-distortionvalue of the four 32×32 CUs obtained in the process 6).

In the intra-prediction mode, a prediction mode is selected as a PUunit, and prediction and reconstruction are performed on the selectedprediction mode in an actual TU unit.

A TU means a basic unit in which actual prediction and reconstructionare performed. A TU includes a transform block (TB) for a luma componentand a TB for two chroma components corresponding to the luma component.

In the example of FIG. 3, as in an example in which one CTU is split inthe quad-tree structure to generate a CU, a TU is hierarchically splitfrom one CU to be coded in the quad-tree structure.

TUs split from a CU may be split into smaller and lower TUs because a TUis split in the quad-tree structure. In HEVC, the size of a TU may bedetermined to be as one of 32×32, 16×16, 8×8 and 4×4.

Referring back to FIG. 3, the root node of a quad-tree is assumed to berelated to a CU. The quad-tree is split until a leaf node is reached,and the leaf node corresponds to a TU.

This is described in more detail. A CU corresponds to a root node andhas the smallest depth (i.e., depth=0) value. A CU may not be splitdepending on the characteristics of an input image. In this case, the CUcorresponds to a TU.

A CU may be split in a quad-tree form. As a result, lower nodes having adepth 1 (depth=1) are generated. Furthermore, a node (i.e., leaf node)that belongs to the lower nodes having the depth of 1 and that is nolonger split corresponds to a TU. For example, in FIG. 3(b), a TU(a), aTU(b) and a TU(j) corresponding to the nodes a, b and j are once splitfrom a CU and have a depth of 1.

At least one of the nodes having the depth of 1 may be split in aquad-tree form again. As a result, lower nodes having a depth 2 (i.e.,depth=2) are generated. Furthermore, a node (i.e., leaf node) thatbelongs to the lower nodes having the depth of 2 and that is no longersplit corresponds to a TU. For example, in FIG. 3(b), a TU(c), a TU(h)and a TU(i) corresponding to the node c, h and I have been split twicefrom the CU and have the depth of 2.

Furthermore, at least one of the nodes having the depth of 2 may besplit in a quad-tree form again. As a result, lower nodes having a depth3 (i.e., depth=3) are generated. Furthermore, a node (i.e., leaf node)that belongs to the lower nodes having the depth of 3 and that is nolonger split corresponds to a CU. For example, in FIG. 3(b), a TU(d), aTU(e), a TU(f) and a TU(g) corresponding to the nodes d, e, f and g havebeen three times split from the CU and have the depth of 3.

A TU having a tree structure may be hierarchically split withpredetermined maximum depth information (or maximum level information).Furthermore, each spit TU may have depth information. The depthinformation may include information about the size of the TU because itindicates the split number and/or degree of the TU.

Information (e.g., a split TU flag “split_transform_flag”) indicatingwhether a corresponding TU has been split with respect to one TU may betransferred to the decoder. The split information is included in all ofTUs other than a TU of a minimum size. For example, if the value of theflag indicating whether a TU has been split is “1”, the corresponding TUis split into four TUs. If the value of the flag indicating whether a TUhas been split is “0”, the corresponding TU is no longer split.

FIG. 5 is an embodiment to which the present invention is applied and isa diagram illustrating an intra-prediction method.

Referring to FIG. 5, the decoder derives an intra-prediction mode of acurrent processing block (S501).

An intra-prediction mode may have a prediction direction for thelocation of a reference sample used for prediction depending on aprediction mode. An intra-prediction mode having a prediction directionis called an intra-angular prediction mode (Intra_Angular predictionmode). In contrast, an intra-prediction mode not having a predictiondirection includes an intra-planar (INTRA_PLANAR) prediction mode and anintra-DC (INTRA_DC) prediction mode.

Table 1 illustrates intra-prediction modes and associated names, andFIG. 6 illustrates a prediction direction according to anintra-prediction mode.

TABLE 1 INTRA-PREDICTION MODE ASSOCIATED NAMES 0 Intra-planar(INTRA_PLANAR) 1 Intra-DC (INTRA_DC) 2 . . . , 34 intra-angular 2 . . ., intra-angular 34 (INTRA_ANGULAR2 . . . , INTRA_ANGULAR34)

In intra-prediction, prediction is performed on a current processingblock based on a derived prediction mode. A reference sample used forprediction and a detailed prediction method are different depending on aprediction mode. If a current block is an intra-prediction mode, thedecoder derives the prediction mode of a current block in order toperform prediction.

The decoder checks whether neighboring samples of the current processingblock can be used for prediction and constructs reference samples to beused for the prediction (S502).

In intra-prediction, neighboring samples of the current processing blockmean a sample neighboring the left boundary of current processing blockof an nS×nS size, a total of 2×nS samples neighboring a bottom left ofthe current processing block, a sample neighboring the top boundary ofthe current processing block, a total of 2×nS samples neighboring thetop right of the current processing block, and one sample neighboringthe top left of the current processing block.

However, some of the neighboring samples of the current processing blockhave not yet been coded or may not be available. In this case, thedecoder may construct reference samples to be used for prediction bysubstituting unavailable samples with available samples.

The decoder may perform filtering on the reference samples based on theintra-prediction mode (S503).

Whether or not to perform the filtering of the reference samples may bedetermined based on the size of the current processing block.Furthermore, the filtering method of the reference samples may bedetermined based on a filtering flag transferred by the encoder.

The decoder generates a prediction block for the current processingblock based on the intra-prediction mode and the reference samples(S504). That is, the decoder generates a prediction block (i.e.,generates a prediction sample) for the current processing block based onthe intra-prediction mode derived in the step S501 of deriving anintra-prediction mode and the reference samples obtained through thereference sample construction step S502 and the reference samplefiltering step S503.

If the current processing block has been encoded in the INTRA_DC mode,in order to minimize the discontinuity of a boundary between processingblocks, a sample at the left boundary of a prediction block (i.e., asample within a prediction block neighboring the left boundary) and asample at the top boundary of the prediction block (i.e., a samplewithin a prediction block neighboring the top boundary) may be filteredat step S504.

Furthermore, at step S504, with respect to the vertical mode andhorizontal mode of intra-angular prediction modes, as in the INTRA_DCmode, filtering may be applied to the left boundary sample or the topboundary sample.

This is described in more detail. If a current processing block has beenencoded in the vertical mode or horizontal mode, the value of aprediction sample may be derived based on a reference sample located ina prediction direction. In this case, a boundary sample that belongs tothe left boundary sample and top boundary sample of a prediction blockand that is not located in the prediction direction may neighbor areference sample not used for prediction. That is, the distance from thereference sample not used for prediction may be much closer than thedistance from a reference sample used for prediction.

Accordingly, the decoder may adaptively apply filtering to left boundarysamples or top boundary samples depending on whether an intra-predictiondirection is vertical or horizontal. That is, if the intra-predictiondirection is vertical, the decoder may apply filtering to the leftboundary samples. If the intra-prediction direction is horizontal, thedecoder may apply filtering to the top boundary samples.

In encoding/decoding according to such an intra-prediction mode, thereis a problem in that the accuracy of prediction is reduced as thedistance from reference samples becomes distant. This is described withreference to the following figure.

FIG. 7 is a diagram for illustrating a problem in existing intra-modeprediction.

FIG. 7 illustrates a case where a TU of a 4×4 size has been encoded in avertical intra-prediction mode. In FIG. 7, an arrow indicates aprediction direction.

Referring to FIG. 7, a prediction sample value is derived using areference sample located in a vertical direction.

In this case, prediction samples 702 located in a top boundary within aTU have high prediction accuracy because they are close to referencesamples 701, whereas prediction samples 703 located in a bottom boundarywithin the TU have low prediction accuracy because they are distant fromthe reference samples 701.

FIG. 8 is a diagram for illustrating a problem in existing intra-modeprediction.

FIG. 8 illustrates PUs (i.e., same as CUs) of an 2N×2N size and TUs(i.e., depth=1) of an N×N size.

In intra-prediction encoding of HEVC, as in FIG. 8, TU split isperformed on a CU in a square form, and actual prediction andreconstruction are performed on each of the split square-shaped TUs. Asshown in FIG. 8, if the size of a PU is 2N×2N and a TU depth is 1, abottom-right sample 802 of each TU has low prediction accuracycorresponding to the distance N between a reference sample 801 and aprediction sample with respect to intra-prediction modes according toall of directions in addition to a vertical intra-prediction mode and ahorizontal intra-prediction mode.

Accordingly, the present invention proposes a method of improving theaccuracy of intra-prediction by minimizing the distance between areference sample and a prediction sample in intra-prediction.

In particular, the present invention proposes a method of splitting aprocessing unit in various forms based on each intra-prediction mode andperforming intra-prediction.

That is, there is proposed a method of splitting a processing unit basedon an intra-prediction mode and performing intra-prediction. Preferably,the processing unit may be split in a form orthogonal to the predictiondirection of each intra-prediction mode.

Hereinafter, in describing an embodiment of the present invention, it isassumed that a unit in which intra-prediction and transform areperformed is a transform unit (TU) (or transform block (TB)), atransform unit is split from a coding unit (CU) (or coding block), and aunit in which an intra-prediction mode is determined is a predictionunit (PU) (or prediction block), for convenience of description, butthis is only an example and the present invention is not limitedthereto. That is, as described above, a transform unit/codingunit/prediction unit may be substituted with a processing unit (orprocessing block) having a specific size or form.

FIG. 9 illustrates an intra-prediction mode-based spit method accordingto an embodiment of the present invention.

FIG. 9 illustrates a PU (i.e., identical with a CU) of a 2N×2N size anda TU of a depth 1.

FIG. 9(a) illustrates a method of splitting a TU in a verticalintra-prediction mode, FIG. 9(b) illustrates a method of splitting a TUin a horizontal intra-prediction mode, and FIG. 9(c) illustrates amethod of splitting a TU in a bottom-right (i.e., 135°) (e.g.,INTRA_ANGULAR18 in the example of FIG. 6) intra-prediction mode.

In FIG. 9, an arrow indicates a prediction direction.

In FIG. 9, a coding order of TUs is performed in order of a TU_0, aTU_1, a TU_2 and a TU_3. After one TU is encoded and decoded, it is usedas a reference sample for the encoding of a next TU.

As in FIG. 9(a), in the vertical intra-prediction mode, TUs may be splitfrom a CU in a horizontal direction. As described above, by splittingthe TUs and performing prediction in the direction vertical to thedirection of an intra-prediction mode, the distance between a referencesample 901 a and the furthermost bottom-right prediction sample 902 anin a TU_0 can be reduced to N/2. Even in a TU_1, likewise, if the TU_0is used as a reference pixel after it is encoded and decoded, thedistance between the reference sample and a bottom-right predictionsample furthermost from the reference sample in the TU_1 can be reducedto N/2. Even in the TU_2 and TU_3, the distance between the referencesample and a prediction sample furthermost from the reference sample canbe reduced to N/2 using the same method.

As in FIG. 9(b), in horizontal intra-prediction, TUs may be split from aCU in a vertical direction. As described above, by splitting the TUs inthe direction vertical to the direction of an intra-prediction mode andperforming prediction, the distance between the reference sample 901 band a bottom-right prediction sample 902 b furthermost from thereference sample in a TU_0 can be reduced to N/2. Even in a TU_1,likewise, if a TU₀ is used as a reference pixel after it is encoded anddecoded, the distance between the reference sample and a bottom-rightprediction sample furthermost from the reference sample in the TU_1 canbe reduced to N/2. Even in a TU_2 and TU_3, the distance between thereference sample and a prediction sample furthermost from the referencesample can be reduced to N/2 using the same method.

As in FIG. 9(c), in an intra-prediction mode of a 135° direction, TUsmay be split from a CU in a 45° direction. As described above, bysplitting the TUs in the direction vertical to the direction of anintra-prediction mode and performing prediction, the distance between areference sample 901 c and a bottom-right prediction sample 902 cfurthermost from the reference sample in a TU_0 can be reduced. Even ina TU_1, likewise, if the TU_0 is used as a reference pixel after it isencoded and decoded, the distance between a reference sample and abottom-right prediction sample furthermost from the reference sample inthe TU_1 can be reduced to N/2. Even in a TU_2 and TU_3, the distancebetween the reference sample and a prediction sample furthermost fromthe reference sample can be reduced to N/2 using the same method.

As in the examples of FIGS. 9(a) to 9(c), TUs are split from a CU in thedirection orthogonal to the prediction direction of an intra-predictionmode, but may be split according to a quad-tree method as in aconventional technology. That is, one CU may be split into four TUs of adepth 1.

In FIG. 9, TUs of a depth 1 have been illustrated, for convenience ofdescription, but the present invention is not limited thereto. That is,TUs may be split by applying the same method as that of FIG. 9 to a TUhaving a depth 2 or more. For example, as in the example of FIG. 9(a),in the vertical intra-prediction mode, a TU having a depth 2 may besplit from each TU having a depth 1. As a result, all of the TUs of thedepth 2 may have N/8 in the vertical direction. That is, a method ofsplitting a TU of a depth 1 from a CU may be identically applied to thespit method.

In accordance with an embodiment of the present invention, all of TUs ofthe same depth split from one CU may be split to have the same area. Inother words, TUs of the same depth split from one CU may be split toinclude the same number of samples. For example, in the case of FIG.9(c), all of the TU 0, TU 1, TU 2 and TU 3 may be split to have the samearea or the same number of samples.

By splitting TUs by considering the direction of a prediction mode asdescribed above, intra-prediction performance can be improved becausethe distance between a reference sample and a prediction sample can bereduced.

Furthermore, in FIG. 9, only intra-prediction direction of the verticaldirection, horizontal direction and 135° direction has been illustrated,for convenience of description, but the present invention is not limitedthereto. That is, TUs may be split in the direction vertical to anintra-prediction direction in various intra-prediction directions. Forexample, in the case of HEVC, a total of 35 prediction modes are usedfor intra-prediction. If the present invention is applied to the 33prediction modes that belong to the 35 prediction modes and that havedirectivity, 33 TU split directions (e.g., vertical to theintra-prediction direction) may be determined depending on directionsaccording to the 33 intra-prediction modes.

FIG. 10 illustrates a method of constructing a reference sample for asplit block based on an intra-prediction mode according to an embodimentof the present invention.

FIG. 10 illustrates only reference samples for a TU 0 and a TU 2 foreach intra-prediction mode, for convenience of description, butreference samples for a TU 1 and a TU 3 may be constructed using thesame method.

FIG. 10(a) illustrates reference samples for TUs split in a horizontaldirection according to a vertical intra-prediction mode. FIG. 10(b)illustrates reference sample for TUs split in a vertical directionaccording to a horizontal intra-prediction mode. FIG. 10(c) illustratesreference samples for TUs split in a 45° direction according to theintra-prediction mode of a 135° direction (e.g., INTRA_ANGULAR18 in theexample of FIG. 6).

Referring to FIG. 10(a), reference samples 1001 a and 1002 a for a TUspit in a horizontal direction may include a sample neighboring the leftboundary of the corresponding TU, a sample neighboring the top boundaryof the corresponding TU and a sample neighboring the top left of thecorresponding TU for each TU, that is, a TU 1, a TU 2, a TU 3 and a TU4.

In this case, the number of reference samples may be determined based onthe size of a TU and/or the split form of the TU.

For example, in the case of a TU of a 2N×N/2 size as in FIG. 10(a), atotal number of the reference samples 1001 a and 1002 a of the TU may bedifferent from those of a TU that is split according to an existingsquare quad-tree method and that has the same depth. That is, in thecase of a TU of an N×N size split in a square quad-tree, the referencesamples 1001 a and 1002 a have a total of 4N+1. In contrast, in the caseof a TU of a 2N×N/2 size as in FIG. 10(a), reference samples may have atotal of (3N)+(N/2)+1, that is, the number of samples neighboring thetop boundary of the corresponding TU and the number of samplesneighboring the top right of the corresponding TU is 3N, the number ofsamples neighboring the left boundary of the corresponding TU and thenumber of samples neighboring the bottom left of the corresponding TU isN/2, and the number of samples neighboring the top left of thecorresponding TU is 1.

For another example, as in FIG. 10(a), in the case of a TU of a 2N×N/2size, the reference samples 1001 a and 1002 a may have a total of(3N)+(N/2)+1, that is, the number of samples neighboring the leftboundary of the corresponding TU and the number of samples neighboringthe bottom left of the corresponding TU is N, the number of samplesneighboring the top boundary of the corresponding TU is (2N)+(N/2), thenumber of samples neighboring the top right of the corresponding TU is(2N)+(N/2), and the number of samples neighboring the top left of thecorresponding TU is 1.

Referring to FIG. 10(b), reference samples 1001 b and 1002 b for a TUsplit in a vertical direction may include a sample neighboring the leftboundary of the corresponding TU, a sample neighboring the top boundaryof the corresponding TU, and a sample neighboring the top left of thecorresponding TU for each of TUs TU 1, TU 2, TU 3 and TU 4.

As described above, the number of reference samples may be determinedbased on the size of a TU and/or a split form of the TU.

For example, as in FIG. 10(b), in the case of a TU of an N/2×2N size, atotal number of the reference samples 1001 b and 1002 b of the TU may bedifferent from those of a TU that is split according to a squarequad-tree method and that has the same depth. In the case of the TU ofthe N×N size split in the square quad-tree, the reference samples 1001 band 1002 b has a total of 4N+1. In contrast, in the case of the TU ofthe N/2×2N size as in FIG. 10(b), reference samples may have a total of(3N)+(N/2)+1, that is, the number of samples neighboring the leftboundary of the corresponding TU and the number of samples neighboringthe bottom left of the corresponding TU are 3N, the number of samplesneighboring the top boundary the corresponding TU and the number ofsamples neighboring the top right of the corresponding TU is N/2, andthe number of samples neighboring the top left of the corresponding TUis 1.

For another example, in the case of the TU of the N/2×2N size as in FIG.10(b), the reference samples 1001 b and 1002 b may have a total of(3N)+(N/2)+1, that is, the number of samples neighboring the leftboundary of the corresponding TU and the number of samples neighboringthe bottom left of the corresponding TU are (2N)+(N/2), the number ofsamples neighboring the top boundary of the corresponding TU and thenumber of samples neighboring the top right of the corresponding TU areN, and the number of samples neighboring the top left of thecorresponding TU is 1.

Referring to FIG. 10(c), reference samples for a TU split in a 45°direction may include sample neighboring the boundary of thecorresponding TU for each TU.

That is, in the case of a TU 0 and a TU 1, a reference sample 1001 c mayinclude a sample neighboring the left boundary of a corresponding TU, asample neighboring the top boundary of the corresponding TU, and asample neighboring the top left of the corresponding TU.

In contrast, in the case of a TU 2 and a TU 3, a reference sample 1002 cmay include a sample neighboring the top left boundary of thecorresponding TU, a sample neighboring the right boundary of thecorresponding TU and a sample neighboring the bottom boundary of thecorresponding TU.

FIG. 10(c) illustrates only the TUs spit in the 45° direction, butreference samples may be constructed according to the same method asthat of FIG. 10(c) in the case of a prediction direction other than avertical direction and a horizontal direction.

As described above, the number of reference samples may be determinedbased on the size of a TU and/or a split form of the TU.

For example, in the case of the TU split as in FIG. 10(c), a totalnumber of the reference samples 1001 c and 1002 c of the TU may bedifferent from those of a TU that is split according to a squarequad-tree method and that has the same depth. That is, the total numberof reference samples 1001 c and 1002 c may be different from those of aTU of an N×N size split in a square quad-tree.

For another example, in the case of the TU split as in FIG. 10(c), thenumber of reference samples may include only samples neighboring theboundaries of the corresponding TU. Accordingly, the number of referencesample may be determined depending on the boundary (e.g., the length ofthe boundary) of a TU that reference samples neighbor.

FIG. 11 is a diagram for illustrating a comparison between an existingblock split method and the block intra-prediction mode-based spit methodaccording to the present invention.

FIG. 11 illustrates split forms of a TU according to a split depth ofthe TU if the size of a CU is 2N×2N and the size of a PU is 2N×2N likethe CU.

FIG. 11(a) illustrates a case where a TU is split according to anexisting square quad-tree manner, and FIG. 11(b) illustrates a casewhere a TU is split based on an intra-prediction mode according to thepresent invention.

As described above, in an intra-prediction mode, the intra-predictionmode is determined in a PU unit, and prediction and reconstruction maybe performed in a TU unit. In this case, prediction and reconstructionare performed on TUs included in the PU according to the sameintra-prediction mode.

Accordingly, as in FIG. 11, if the prediction mode (PredMode) of a PU isA, prediction and reconstruction are performed on all of TUs based onthe same prediction mode (PredMode) A regardless of whether the TUs aresplit from a CU according to the existing square quad-tree method orwhether the TUs are split in the direction vertical to anintra-prediction direction according to the present invention. In FIG.11, the prediction mode (PredMode) A is assumed to be a horizontalintra-mode prediction mode.

In accordance with a TU split method according to the present invention,a split direction is determined depending on the direction of anintra-prediction mode, but the number of split TUs may be the same asthat of an existing square quad-tree method. That is, in the case of aTU of a depth 1, one CU is split into four TUs. In the case of a TU of adepth 2, one CU is split into 16 TUs.

That is, even in the TU split method according to the present invention,a quad-tree method may be applied. Accordingly, whenever a depth isincreased by 1, one CU (or TU) may be split into four TUs of a lowerlevel.

FIG. 12 is a more detailed diagram illustrating the intra-predictionunit according to an embodiment of the present invention.

Referring to FIG. 12, the intra-prediction unit (refer to 182 of FIGS. 1and 262 of FIG. 2) implements the functions, processes and/or methodsproposed in FIGS. 7 to 11. Specifically, the intra-prediction unit 182,262 may be configured to include a TU split unit 1202 and anintra-prediction processing unit 1203.

Furthermore, the intra-prediction unit 182, 262 may be configured tofurther include a TU split method determination unit 1201.

The TU split method determination unit 1201 determines a spit methodbased on whether the spit method of a current TU (or TB) is an existingsquare quad-tree spit method or an intra-prediction mode.

In this case, a spit method of a current TU (or TB) may be determinedbased on an intra-prediction mode. For example, as in Table 1, ifintra-prediction modes have been defined, a spit method of a TU (or TB)to which the intra-prediction mode (i.e., 0 and 1) not havingdirectivity has been applied may be determined to be the squarequad-tree spit method. A spit method of a TU (or TB) to which theintra-prediction modes (i.e., 2 to 34) having directivity has beenapplied may be determined to be a spit method based on anintra-prediction mode.

If a TU spit method has not been determined for each TU (or TB) (e.g.,if only an intra-prediction mode-based spit method is applied to all ofcurrent pictures), the TU split method determination unit 1201 may notbe included in the intra-prediction unit 182, 262.

The TU split unit 1202 may split a current TU (or TB) based on anintra-prediction mode. In this case, the TU split unit 1202 may splitthe current TU (or TB) according to the quad-tree method in thedirection orthogonal to the prediction direction of the intra-predictionmode of the current TU (or TB) as in FIGS. 9 to 11.

In this case, the decoder may determine whether or not to split thecurrent TU (or TB) using a split flag provided by the encoder.

Furthermore, the TU split unit 1202 may split the current TU (or TB)according to the existing square quad-tree spit method.

The intra-prediction processing unit 1203 performs intra-prediction oneach of TUs (or TBs).

The intra-prediction processing unit 1203 may perform intra-predictionon a current TU (or TB) using the process according to the example ofFIG. 5. In this case, reference samples may be configured according tothe example of FIG. 10.

FIG. 13 is a diagram illustrating a method of processing a video signalbased on an intra-prediction mode according to an embodiment of thepresent invention.

Referring to FIG. 13, the decoder (in particular, the intra-predictionunit) determines whether the split flag of a current TU (or TB) is 1(S1301).

In this case, the current TU (or TB) may be specified because a locationvalue (e.g., coordinate value) for specifying the current TU (or TB) isset as a location value of the top-left sample of the current TU (orTB).

In this case, the split flag may be provided as a syntax element by theencoder.

If the split flag is 1 at step S1301, the decoder splits the current TU(or TB) based on an intra-prediction mode (S1302).

That is, as in the examples of FIGS. 9 to 11, the decoder may split thecurrent TU (or TB) in a form orthogonal to the prediction direction ofthe intra-prediction mode.

In this case, to split the current TU (or TB) may mean that the locationvalue (e.g., coordinate value) for specifying the current TU (or TB) isset as a location value of a specific sample (e.g., a location value ofthe top-left sample) of a spit TU (or TB) based on the intra-predictionmode.

Accordingly, the spit TU (or TB) corresponds to the current TU (or TB),and step S1301 is performed. Furthermore, steps S1301 and S1302 arerepeatedly performed until the split flag of the current TU (or TB) isnot 1.

If the split flag is 0 at step S1301, the decoder performsintra-prediction on the current TU (or TB) based on the intra-predictionmode (S1303).

In this case, the decoder may perform intra-prediction on the current TU(or TB) using the process according to the example of FIG. 5. In thiscase, the reference sample may be constructed based on the example ofFIG. 10.

Meanwhile, the encoder (in particular, the intra-prediction unit) mayperform the same process except step S1301 of FIG. 13. That is, theencoder may split a current TU (or TB) based on an intra-prediction modeand perform intra-prediction.

FIG. 14 is a diagram illustrating a method of processing a video signalbased on an intra-prediction mode according to an embodiment of thepresent invention.

Referring to FIG. 14, the decoder (in particular, the intra-predictionunit) determines a spit method of a current TU (or TB) (S1401).

In this case, the spit method of the current TU (or TB) may bedetermined based on an intra-prediction mode. For example, ifintra-prediction modes have been defined as in Table 1, the spit methodof a TU (or TB) to which the intra-prediction modes (i.e., 0 and 1) nothaving directivity have been applied may be determined to be a squarequad-tree spit method, and the spit method of a TU (or TB) to which theintra-prediction modes (i.e., 2 to 34) having directivity have beenapplied may be determined to be an intra-prediction mode-based spitmethod.

In this case, the current TU (or TB) may be specified because a locationvalue (e.g., coordinate value) for specifying the current TU (or TB) isset as a location value of the top-left sample of the current TU (orTB).

If the spit method of the current TU (or TB) is a square quad-tree spitmethod at step S1404, the decoder determines whether the split flag ofthe current TU (or TB) is 1 (S1402).

In this case, the split flag may be provided as a syntax element by theencoder.

If the split flag is 1 at step S1402, the decoder splits the current TU(or TB) according to a square quad-tree method (S1403).

That is, as in the example of FIG. 3, the decoder may split the currentTU (or TB) according to the square quad-tree method.

In this case, to split the current TU (or TB) may mean that the locationvalue (e.g., coordinate value) for specifying the current TU (or TB) isset as the location value of the top-left sample of a spit TU (or TB)according to the square quad-tree method.

Accordingly, the spit TU (or TB) corresponds to the current TU (or TB),and step S1402 is performed. Furthermore, steps S1402 and S1403 arerepeated until the split flag of the current TU (or TB) is not 1.

In contrast, if the spit method of the current TU (or TB) is anintra-prediction mode-based spit method at step S1401, the decoderdetermines whether the split flag of the current TU (or TB) is 1(S1404).

In this case, the split flag may be provided as a syntax element by theencoder.

If the split flag is 1 at step S1404, the decoder splits the current TU(or TB) based on an intra-prediction mode (S1302).

That is, as in the examples of FIGS. 9 to 11, the decoder may split thecurrent TU (or TB) in a form orthogonal to the prediction direction ofthe intra-prediction mode.

In this case, to split the current TU (or TB) may mean that the locationvalue (e.g., coordinate value) for specifying the current TU (or TB) isset as the location value of a specific sample of the spit TU (or TB)(e.g., the location value of the top-left sample) based on theintra-prediction mode.

Accordingly, the spit TU (or TB) corresponds to the current TU (or TB),and step S1404 is performed. Furthermore, steps S1404 and S1405 arerepeated until the split flag of the current TU (or TB) is not 1.

Meanwhile, if the split flag is 0 at steps S1402 or S1404, the decoderperforms intra-prediction on the current TU (or TB) based on anintra-prediction mode (S1406).

In this case, the decoder may perform intra-prediction on the current TU(or TB) using the process according to the example of FIG. 5.

In this case, if the current TU (or TB) has been split according to anintra-prediction mode-based spit method, a reference sample may beconstructed based on the example of FIG. 10.

Meanwhile, the encoder (in particular, the intra-prediction unit) mayperform the same process except steps S1402 and S1404 of FIG. 14. Thatis, a spit method of a current TU (or TB) may be determined, the currentTU (or TB) may be split according to the spit method, andintra-prediction may be performed on the current TU (or TB).

FIG. 15 is a diagram for illustrating a method of relocating (orreconstructing) a transform unit according to an embodiment of thepresent invention.

FIG. 15 shows an example in which an intra-mode prediction direction isvertical and split from a CU of a 2N×2N size into four TUs having adepth 1 is performed in a horizontal direction.

If the split of TUs is performed based on an intra-prediction modeproposed by the present invention, it is difficult to apply transformprovided in HEVC. That is, in HEVC, transform to a TU unit of a squareform is performed. However, according to a TU split method according tothe present invention, it is difficult to apply transform provided inHEVC because a TU split form is determined based on an intra-predictiondirection.

Accordingly, TUs split in a form orthogonal to the intra-predictiondirection are relocated (or reconstructed) so that the transform of HEVCcan be applied to the spit TUs.

Referring to FIG. 15, a TU 0 1520 of an N×N size of a square form isconstructed by splitting a TU 0 1510 having a 2N×N/2 size into twoblocks 1511 and 1512 each having a half horizontal size and relocating(or reconstructing) them in a vertical direction.

In this case, the TUs may be relocated in predetermined scan order. Forexample, in accordance with raster scan order, since a decoding processis performed on the left TU 0 1511 earlier than the right TU 0 1512, theleft TU 0 1511 may be disposed on the upper side 1521 and the right TU 01512 may be disposed on the lower side 1522 when the TUs are relocatedfor transform.

The decoder performs the same process on the remaining TU 1, TU 2 and TU3 using the same method.

As described above, the decoder performs prediction and reconstructionin a spit TU unit based on each intra-prediction mode. Furthermore, fortransform, the TUs are relocated in a square block according topredefined scan order, and transform is performed in a relocated TUunit.

FIG. 16 is a diagram for illustrating a comparison between an existingtransform block split method and a transform block reconstruction methodaccording to the present invention.

FIG. 16 illustrates a split form of TUs in which the size of a CU is2N×2N and a split depth is 1.

FIG. 16(a) illustrates a case where TUs are split according to anexisting square quad-tree method, and FIG. 16(b) illustrates a casewhere TUs are split based on intra-prediction direction according to thepresent invention. In particular, FIG. 16(b) illustrates a case where anintra-mode prediction direction is vertical and four TUs are split in ahorizontal direction.

As in FIG. 16(a), if TUs are split according to the existing squarequad-tree method, the decoder performs prediction and reconstruction oneach TU of a square form in predefined scan order and also performstransform.

In contrast, as in FIG. 16(b), if TUs are split based on theintra-prediction mode, the decoder performs prediction andreconstruction in a spit TU unit in predefined scan order so that TUsare vertical to the prediction direction of the intra-prediction mode.Furthermore, the decoder relocates (or reconstructs) the spit TUs in TUsof a square form so that they are vertical to the prediction directionand then performs transform.

In FIGS. 15 and 16, a case where the prediction direction of theintra-prediction mode is vertical has been illustrated, but the abovemethod may be identically applied to a case where the predictiondirection is different.

FIG. 17 is a diagram for illustrating a method of relocating (orreconstructing) a TU according to an embodiment of the presentinvention.

FIG. 17 illustrates a case where an intra-mode prediction direction ishorizontal and four TUs having a depth 1 are split from a CU of a 2N×2Nsize in a vertical direction.

As described above, it is difficult to apply transform provided in HEVCbecause a TU spit form is determined based on an intra-predictiondirection. TUs are relocated (or reconstructed) so that transform ofHEVC can be applied to a spit TU in a form orthogonal to theintra-prediction direction.

Referring to FIG. 17, a TU 0 1710 having an N/2×2N size is split intotwo blocks 1711 and 1712) each having a half vertical size. The twoblocks 1711 and 1712 are relocated in a horizontal direction toconstruct a TU 0 1720 of an N×N size of a square form.

In this case, the TU may be relocated in predetermined scan order. Forexample, according to raster scan order, since a decoding process isperformed on the top TU 0 1711 earlier than the bottom TU 0 1712, thetop TU 0 1711 may be disposed in a left 1721 and the bottom TU 0 1712may be disposed in a right 1722 when the TUs are relocated fortransform.

The decoder performs the same process on the remaining TU 1, TU 2 and TU3 using the same method.

As described above, the decoder performs prediction and reconstructionin a spit TU based on each intra-prediction mode. Furthermore, fortransform, the decoder relocates a TU in a square block in predefinedscan order and performs transform in a relocated TU unit.

FIG. 18 is a diagram for illustrating a method of relocating (orreconstructing) a TU according to an embodiment of the presentinvention.

FIG. 18(a) illustrates a case where an intra-mode prediction directionis a 135° direction (e.g., INTRA_ANGULAR18 in the example of FIG. 6) andfour TUs having a depth 1 are split from a CU of a 2N×2N size in a 45°direction.

Referring to FIG. 18, the decoder constructs a TU 0 1820 of a squareform by relocating (or reconstructing) samples, included in a spit TU 01810, based on an intra-prediction mode according to predeterminedorder.

In this case, a TU may be relocated (or reconstructed) in predeterminedscan order. For example, according to raster scan order, samplesincluded in the spit TU 0 1810 may be sequentially disposed in the TU 01820 of a square form from a top-left sample to a bottom-right samplebased on an intra-prediction mode.

Meanwhile, although a prediction direction is vertical or horizontal, asin the example of FIG. 18, a TU of a square form may be constructed byrelocating (or reconstructing) samples included in a corresponding TU ina square form.

FIG. 19 is a more detailed diagram illustrating the transformunit/inverse transform unit according to an embodiment of the presentinvention.

FIG. 19 illustrates the transform unit/inverse transform unit (refer to120/150 of FIG. 1 and refer to 230 of FIG. 2) in one block form, forconvenience of description. However, if the transform unit/inversetransform unit (refer to 120 of FIG. 1 and refer to 230 of FIG. 2) areincluded in the encoder, the transform unit corresponds to inversetransform, and the transform/inverse transform unit 1093 corresponds tothe transform processing unit or the inverse transform processing unit.Likewise, if the transform unit/inverse transform unit 120/230 isincluded in the decoder, they correspond to the inverse transform unit,and the transform/inverse transform unit 1093 corresponds to the inversetransform processing unit.

Referring to FIG. 19, the transform unit/inverse transform unit 120/230implements the functions, processes and/or methods proposed in FIGS. 15to 18. Specifically, the transform unit/inverse transform unit 120/230may be configured to include a TU split determination unit 1901, a TUreconstruction unit 1902 and a transform/inverse transform unit 1903.

The TU split determination unit 1901 determines whether a current TU (orTB) has been split in a square form. That is, the TU split determinationunit 1901 determines whether the current TU (or TB) has been splitaccording to an existing square quad-tree method or split according to aspit method based on an intra-prediction mode.

In this case, the TU split determination unit 1901 may determine a spitmethod based on an intra-prediction mode of the current TU (or TB). Forexample, as in Table 1, if intra-prediction modes have been defined, thespit method of a TU (or TB) to which the intra-prediction modes (i.e., 0and 1) not having directivity have been applied may be determined to bea square quad-tree spit method, and the spit method of a TU (or TB) towhich the intra-prediction modes (i.e., 2 to 34) having directivity havebeen applied may be determined to be an intra-prediction mode-based spitmethod.

The TU reconstruction unit 1902 reconstructs (or relocates) the currentTU (or TB) in a square block. In this case, the TU reconstruction unit1902 may reconstruct (or relocate) the current TU (or TB) in a squareblock using the method of reconstructing (or relocating) a TU (or TB)according to FIGS. 15 to 18.

The transform/inverse transform unit 1903 performs transform/inversetransform processing on the current TU (or TB). In this case, thetransform/inverse transform unit 1903 may transform/inverse transformthe current TU (or TB) using the method described in FIGS. 1 and 2.

FIG. 20 is a diagram illustrating a method of processing a video signalbased on an intra-prediction mode according to an embodiment of thepresent invention.

In FIG. 20, transform/inverse transform steps have been illustrated asbeing one step, for convenience of description, but the encoder mayperform transform or inverse transform and the decoder may performinverse transform.

Referring to FIG. 20, the decoder/encoder (in particular, theintra-prediction unit) determines whether a current TU (or TB) is asquare block (S2001).

In this case, the decoder/encoder may determine a spit method based onan intra-prediction mode of the current TU (or TB). For example, as inTable 1, if intra-prediction modes have been defined, the spit method ofa TU (or TB) to which the intra-prediction modes (i.e., 0 and 1) nothaving directivity have been applied may be determined to be a squarequad-tree spit method, and the spit method of a TU (or TB) to which theintra-prediction modes (i.e., 2 to 34) having directivity have beenapplied may be determined to be an intra-prediction mode-based spitmethod.

If the current TU (or TB) is not a square block at step S2001, thedecoder/encoder reconstructs (or relocates) the current TU (or TB) in asquare block form (S2002).

In this case, the decoder/encoder (in particular, the intra-predictionunit) may reconstruct (or relocate) the current TU (or TB) in a squareblock form using the method of reconstructing (or relocating) a TU (orTB) according to FIGS. 15 to 18.

In contrast, if the current TU (or TB) is a square block at step S2001or after the current TU (or TB) is reconstructed (or relocated) in asquare block form at step S2002, the decoder/encoder performstransform/inverse transform on the current TU (or TB) (S2003).

The decoder/encoder may transform/inverse transform on the current TU(or TB) using the method described in FIGS. 1 and 2.

In the aforementioned embodiments, the elements and characteristics ofthe present invention have been combined in specific forms. Each of theelements or characteristics may be considered to be optional unlessexplicitly described otherwise. Each of the elements or characteristicsmay be implemented in a form not combined with another element orcharacteristic. Furthermore, some of the elements and/or thecharacteristics may be combined to form an embodiment of the presentinvention. The order of the operations described in connection with theembodiments of the present invention may be changed. Some of theelements or characteristics of an embodiment may be included in anotherembodiment or may be replaced with corresponding elements orcharacteristics of another embodiment. It is evident that an embodimentmay be constructed by combining claims not having an explicit citationrelation in the claims or may be included as a new claim by amendmentsafter filing an application.

An embodiment of the present invention may be implemented by variousmeans, for example, hardware, firmware, software or a combination ofthem. In the case of implementations by hardware, an embodiment of thepresent invention may be implemented using one or moreapplication-specific integrated circuits (ASICs), digital signalprocessors (DSPs), digital signal processing devices (DSPDs),programmable logic devices (PLDs), field programmable gate arrays(FPGAs), processors, controllers, microcontrollers and/ormicroprocessors.

In the case of an implementation by firmware or software, an embodimentof the present invention may be implemented in the form of a module,procedure, or function for performing the aforementioned functions oroperations. Software code may be stored in memory and driven by aprocessor. The memory may be located inside or outside the processor,and may exchange data with the processor through a variety of knownmeans.

It is evident to those skilled in the art that the present invention maybe materialized in other specific forms without departing from theessential characteristics of the present invention. Accordingly, thedetailed description should not be construed as being limitative fromall aspects, but should be construed as being illustrative. The scope ofthe present invention should be determined by reasonable analysis of theattached claims, and all changes within the equivalent range of thepresent invention are included in the scope of the present invention.

INDUSTRIAL APPLICABILITY

As described above, the preferred embodiments of the present inventionhave been disclosed for illustrative purposes, and those skilled in theart may improve, change, substitute, or add various other embodimentswithout departing from the technological spirit and scope of the presentinvention disclosed in the attached claims.

1. A method of processing an image based on an intra-prediction mode,comprising steps of: splitting a processing block based on anintra-prediction mode of the processing block; and performingintra-prediction on the split processing block, wherein a splitdirection of the split processing block is vertical to a predictiondirection of the intra-prediction mode of the processing block.
 2. Themethod of claim 1, wherein the processing block is split when a splitflag of the processing block is
 1. 3. The method of claim 1, whereinwhether the processing block is split according to a square quad-treemethod or split vertically to the prediction direction is determinedbased on the intra-prediction mode of the processing block.
 4. Themethod of claim 3, wherein: the processing block is split according tothe square quad-tree method if the intra-prediction mode of theprocessing block is intra-planar or intra-DC, otherwise, the processingblock is split vertically to the prediction direction.
 5. The method ofclaim 1, further comprising steps of: reconstructing the splitprocessing block into a square block; and performing transform/inversetransform on the reconstructed processing block.
 6. The method of claim5, wherein if the split processing block is 2N×N/2, the split processingblock is split into two blocks each having a half horizontal size andthe two blocks are vertically relocated to reconstruct a square N×Nblock.
 7. The method of claim 5, wherein if the split processing blockis N/2×2N, the split processing block is split into two blocks eachhaving a half vertical size and the two blocks are horizontallyrelocated to reconstruct a square N×N block.
 8. The method of claim 5,wherein samples included in the split processing block are relocated inpredetermined order to reconstruct the square block.
 9. The method ofclaim 1, further comprising a step of constructing a reference samplefor the split processing block.
 10. The method of claim 9, wherein if asplit direction of the split processing block is horizontal or vertical,the reference sample comprises a sample neighboring a left boundary ofthe split processing block, a sample neighboring a top boundary of thesplit processing block and a sample neighboring a top left of the splitprocessing block.
 11. The method of claim 9, wherein if the splitdirection of the processing block is 45°, the reference sample comprisesa sample neighboring a left boundary of the split processing block, asample neighboring a top boundary of the split processing block, and asample neighboring a top left of the split processing block.
 12. Themethod of claim 9, wherein if the split direction of the processingblock is 45°, the reference sample comprises a sample neighboring a topleft boundary of the split processing block, a sample neighboring aright boundary of the split processing block, a sample neighboring abottom boundary of the split processing block.
 13. An apparatus forprocessing an image based on an intra-prediction mode, comprising: asplit unit splitting a processing block based on an intra-predictionmode of the processing block; and an intra-prediction processing unitperforming intra-prediction on the split processing block, wherein asplit direction of the split processing block is vertical to aprediction direction of the intra-prediction mode of the processingblock.